Powershell - Get yesterday's file name in real time - powershell

Question

I'm noob in PowerShell and trying to use it to send email with attachment.
Today's date is 2021-02-18. I have a folder which have many files created by ERP system everyday, the files name are as follows:
2021-2-17WO_rate.xlsx
2021-2-16WO_rate.xlsx
2021-2-15WO_rate.xlsx
2021-2-14WO_rate.xlsx
2021-2-13WO_rate.xlsx
2021-2-12WO_rate.xlsx
...
I would like the script always choose the newest file as the attachment
(Get-Date).AddDays(-1).ToString('yyyy-MM-dd') can get the yesterday date, but when I intergrate it in to below script, it fails. I have no idea how to solve it after search the many websites.
Anyone can help me? much appreciate.
The code I use is as follows:
$UserName = "XXXX#XXXXX.com"
$Password = ConvertTo-SecureString XXXXXX -AsPlainText –Force
$cred = New-Object System.Management.Automation.PSCredential($UserName,$Password)
$a = (Get-Date).AddDays(-1).ToString('yyyy-MM-dd')
$mailParams = #{
SmtpServer = 'smtp.office365.com'
Port = '587' # or '25' if not using TLS
UseSSL = $true ## or not if using non-TLS
BodyasHtml = $true
Credential = $cred
From = $UserName
To = 'XXXXXX'
Subject = "WORK ORDER STATUS"
Body = 'XXXXXXX'
Attachments = '\\us-fs\us-Groups\Operationsus\Public\erpoutput\date\${a}WO_rate.xlsx'
DeliveryNotificationOption = 'OnFailure', 'OnSuccess'
}
Send-MailMessage #mailParams

Answer 1

The reason why the name is not populated is because you used single quotation mark '. If you want to have the value from the variable, you need to use double quotation string ":
<# Only relevant parts of the script, other lines removed #>
# Specify correct formatting string, based on the information from the question it should be either
$dataFormat = 'yyyy-M-d'
# or
$dataFormat = 'yyyy-M-dd'
$formattedDate = (Get-Date).AddDays(-1).ToString($dataFormat)
$mailParams = #{
<# other params #>
Attachments = "\\us-fs\us-Groups\Operationsus\Public\erpoutput\date\${formattedDate}WO_rate.xlsx"
}
Send-MailMessage #mailParams
NOTE: I also changed variable name so that it's obvious what it is used for.
Edit: corrected date format based on #Theo's comment

Answer 2

If the file is not modified since yesterday as well you can retrieve the path of the last file modified yesterday in the folder and use that as the attachment
$file = Get-ChildItem -Path 'C:\path\to\folder' -Filter '*.xlxs' -File | # get all files with xlxs extension in path
Where-Object { $_.LastWriteTime.Date -eq (Get-Date).AddDays(-1).Date } | # files that were last written to yesterday
Sort-Object LastWriteTime | # sort by lastwrite time in ascending order
Select-Object -Last 1 | # select only the last file on the list
ForEach-Object FullName # output the full file path
$UserName = "XXXX#XXXXX.com"
$Password = ConvertTo-SecureString XXXXXX -AsPlainText –Force
$cred = New-Object System.Management.Automation.PSCredential($UserName,$Password)
$a = (Get-Date).AddDays(-1).ToString('yyyy-MM-dd')
$mailParams = #{
SmtpServer = 'smtp.office365.com'
Port = '587' # or '25' if not using TLS
UseSSL = $true ## or not if using non-TLS
BodyasHtml = $true
Credential = $cred
From = $UserName
To = 'XXXXXX'
Subject = "WORK ORDER STATUS"
Body = 'XXXXXXX'
Attachments = $file # <--------------
DeliveryNotificationOption = 'OnFailure', 'OnSuccess'
}
Send-MailMessage #mailParams